Harmonic correction in phase-locked loops

ABSTRACT

Correction for harmonic disturbances on rotating storage media in a phase-locked loop. The effects of harmonic disturbances in a phase-locked loop are reduced by employing harmonic correction. Harmonic correction may be present in the loop at all times, or may be switched in once the loop has achieved phase lock. Disturbance within the loop bandwidth is corrected using additional integrating pole or a bump (resonant) filter. Disturbance outside the loop bandwidth is corrected using low pass or a notch (anti-resonant) filter. Alternately, a canceling signal may be generated and added as a feedforward signal. A repetitive control scheme uses a filtered version of the residual errors on previous media rotations as a feedforward signal to cancel harmonic effects.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This is a divisional of application Ser. No. 09/536,298 filed on Mar.27, 2000, now U.S. Pat. No. 6,646,964, which is hereby incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of data storage andsynchronization. In particular it relates to a method for compensatingfor a class or repeatable disturbances that are very common in the areaof rotating machinery, and specifically in the area of rotating storagemedia on a spindle. In such cases, spindle rotation causes manydisturbances to be injected into the loop used to recover signals usedfor synchronization. A large fraction of these disturbances areharmonic, that is, they occur at a known frequency which is related tothe spindle frequency.

2. Art Background

Disk data storage devices feature rotating media with data recorded ontracks on the media. These tracks may be in the form of a plurality ofconcentric circles, or they may be in the form of a single spiral.Formatting information is present on the media which allows the diskdrive to recover the signals needed to read and possibly writeinformation to and from the media. The media is rotated on a spindle.Imperfections in the spindle apparatus, and in the positioning of themedia, introduce disturbances into signals read from the media. Many ofthese disturbances are harmonic in nature, occurring at a knownfrequency which is related to the spindle frequency.

For fixed magnetic disk drives, in which the tracks are formatted afterthe media has been secured to the spindle, the dominant harmonics aremost closely related to minor defects and tolerances in the spindleitself. However, in removable media, such as an optical storage medium,including but not limited to DVD+RW, the dominant feature is theimprecise positioning of the media and therefore the tracks relative tothe true center of the spindle. This positioning offset causeseccentricity in the path which the tracks will take. This means thetracks will not pursue a true circle around the axis of rotation of thespindle; instead, the tracks will have an eccentricity which manifestsitself as a set of sinusoidal deviations from the true circular path.

In a storage device using a rotating storage medium, the act of readingor writing data necessitates the generation of a clock signal to keepthe data synchronized. Furthermore, this clock must be synchronized tothe rotating medium itself, so that the data can be repeatablypositioned on, and recovered from, the storage medium. In order togenerate a clock for reading or writing, it is common to use aphase-locked loop (PLL) which generates a repeatable clock which uses asits input a reference signal measured from the rotating medium.Phase-locked loops have the general nature that they are feedback loopsapplied to electronic signals rather than motion control signals.

A multiplicity of loops are commonly used in rotating storage devices.One loop maintains the tracking position of the read/write assembly.Another loop produces the reference clock used for reading and writingdata. A third possible loop is used in far field devices, such asrewriteable optical storage, to maintain the height of the readbackmechanism or the focus position of the objective lens. Both of theformer loops exhibit sensitivity to track eccentricity. The sensitivityof the tracking position loop to the harmonic disturbances describedabove can be reduced using a variety of methods known to the art.However, even if the distorted track were being followed perfectly, themere act of following the eccentricity would produce differences in thereference clock period around the circumference of the track.

What is needed is a method for correcting clock recovery loops in thepresence of harmonic disturbances.

SUMMARY OF THE INVENTION

Disturbances introduced into a phase-locked loop (PLL) by harmonicsources in rotating storage media are reduced by applying harmoniccorrection. Harmonic correction may be present at all times in the PLL,or may be switched in once loop lock has been obtained. Harmoniccorrection reduces the resultant noise and jitter of the loop. Thenature of the harmonic correction employed depends on the nature of thedisturbance, as well as the nature of the loop. In a first embodiment,where the disturbance is well within the PLL bandwidth, an additionalintegrating pole or a bump (or resonant) filter is added to the loop. Ina second embodiment, where the disturbance is well outside the PLLbandwidth, an additional low pass or notch (anti-resonant) filter isadded to the loop. In a third embodiment, harmonic correction isobtained by generating a sinusoid or a combination of sinusoids at aphase and frequency so as to cancel out the disturbance; this signal isadded as a feedforward signal. In a forth embodiment, harmoniccorrection is obtained in a repetitive control scheme using a filteredversion of the residual errors on previous rotations of the media as afeedforward signal to cancel harmonic effects. These embodiments may berepeated for each harmonic frequency at which a significant disturbanceis present, and may be used in combination with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with respect to particular exemplaryembodiments thereof and reference is made to the drawings in which:

FIG. 1 shows the effects of track eccentricity in a disk drive,

FIG. 2 shows an optical disk drive system with a phase-locked loop,

FIG. 3 shows a harmonic locking phase-locked loop using a phasedetector,

FIG. 4 shows a mixing Costas loop,

FIG. 5 shows a mixing loop with harmonic correction,

FIG. 6 shows a harmonic locking phase-locked loop with harmoniccorrection,

FIG. 7 shows a mixing Costas loop with harmonic correction,

DETAILED DESCRIPTION

In removable media storage devices, imprecise positioning of the disk onthe rotating spindle introduces errors. As shown in FIG. 1(a), diskmedia 100 has a circular track 110 and disk center 120. The center ofdisk 100, of track 110, and disk center 120, are all coincident.Unfortunately, when the media is clamped in place on the spindle of adisk drive, the center of the spindle may not be coincident with thecenter of the disk; the two centers may be offset. Each time the mediais removed and replaced, this misalignment may change. The effect ofthis misalignment is shown in exaggerated form in FIG. 1(b). Here, media100 with its preformatted track 110 and disk center 120 is shownpositioned off the center of spindle 130. The apparent track is shown as140. This misalignment produces a sinusoidal variation in the trackmotion as the media rotates. To this is added errors in spindlerotation, which introduces higher order distortion terms. It has beenfound that the first harmonic is due mostly to misalignment of the diskcenter with the spindle center and is dominant. However, other harmonicsmay be of interest, and may be compensated for in similar fashion inaccordance with the present invention.

FIG. 2 shows an optical disk drive system using a phase-locked loop torecover a clock signal. Optical disk 100 is illuminated by laserassembly 110, which produces and focuses a spot of light on the disk.The beam from laser 110 passes through beam splitter 120, which directslight reflected from disk 100 to mirror 130 and detector assembly 140.One of the outputs of detector 140 is a signal 150 used to recover areference signal. Signal 150 is passed to phase-locked loop 200 toregenerate the reference signal.

Signal 150 is first passed through band pass filter 210, reducing oreliminating frequencies outside of the band of interest. In a mixingloop, the output of bandpass filter 210 is passed to mixer 220. Theoutput of mixer 220 is passed through lowpass filter 230 and loop filter240, and is used to control voltage controlled oscillator (VCO) 250. Oneoutput of VCO 250 is a clock signal 260 which is phase-locked to thereference signal recovered from disk 100. This signal 270 is also sentto mixer 220. The mixer accomplishes the task of phase detection.However, phase detection can also be accomplished in a variety of waysas known to the art, such as those described in Wolaver's Phase LockedLoop Circuit Design Advanced Reference Series & Biophysics andBioengineering Series, published by Prentice Hall, pp 47-79. In many ofthese approaches, the sinusoidal characteristic of the clock signal isreplaced with a rectangular or square wave signal.

Other phase-locked loop topologies are also used for such signalrecovery. FIG. 3 shows a harmonic locking loop using a phase detector.This topology is useful in that it is capable of producing a referencesignal 370 which is a multiple of input signal 150.

Input signal 150 first passes through bandpass filter 310, and issquared up by limiter 320. The output of limiter 320 is one of theinputs to phase detector 330. The output of phase detector 330 is fed tolowpass filter 340 and loop filter 350, which controls voltagecontrolled oscillator 360. Signal 370 is the reference signal output.The output of VCO 360 is also passed through divider 380, and thisdivided down signal 390 forms the other input to phase detector 330.Thus output signal 370 is phase-locked to input signal 150, and is amultiple of that input signal, as determined by divider 380. Note thatdivider 380 may provide a non-integer factor in some applications.

FIG. 4 shows a mixing Costas loop. As before, signal 150 goes throughbandpass filter 410. In the Costas loop, the output of bandpass filter410 drives quadrature mixer 420 and in-phase mixer 430. The output ofmixer 420 is low pass filtered 440 and fed to mixer 460, and the outputof mixer 430 is low pass filtered 450 and fed to mixer 460. The outputof mixer 460 feeds loop filter 470, which controls voltage controlledoscillator 480. The output of VCO 480 fed to quadrature mixer 420, anddelayed 490 one quarter cycle (−π/2), to feed in phase mixer 430.Delayed signal 500 is the phase-locked reference signal. In thistopology, a VCO having quadrature outputs may also be used, and phasedetectors may be substituted for mixers.

Regardless of the PLL topology used, whether they use mixers or phasedetectors, whether they are digital or analog, or whether they areconventional PLLS, harmonic locking, or Costas loops, analog or digital,the goal is to reduce the effects of the eccentricity introduced by diskmisalignment. This disk misalignment produces harmonic errors in theinput signal, which in turn produces phase errors in the referencesignal output by the PLL, even when the loop is locked. Because thephase-locked loop is a nonlinear device which behaves linearly only nearlock, harmonic correction as taught by the present invention is distinctfrom known loop filters. The application of harmonic correction totiming applications, such as phase locked loops, is new.

Harmonic correction solutions take the form of filtering solutions,where the harmonic is filtered out, or cancellation and feedforwardsolutions. In all embodiments, harmonic correction may be applied to theloop at all times, or it may be switched in once the desired degree ofloop lock has been obtained. Such a switching strategy can provide foraggressive locking performance of the loop, followed by the reducedphase jitter achieved when harmonic correction is enabled.

In dealing with a harmonic disturbance, the disturbance can be removedfrom the input signal, and not passed to the PLL. This is a form ofexternal harmonic correction, since the harmonic disturbance never getsinto the loop. External filtering is applied by placing a notch filterfor each desired harmonic component ahead of the PLL input bandpassfilter, or as part of the PLL input bandpass filter.

Alternatively, the PLL VCO can track the disturbance. This is a form ofinternal harmonic correction. In the case of disk drives, the harmonicdisturbance represents a legitimate change in the reference signal onthe disk itself as the media passes under the read head. Thus it may beadvantageous to follow this harmonic signal, so that the VCO outputtracks the harmonic disturbance. This results in a bettersynchronization between the recovered signal from the media, and thereference generated by the PLL.

In a first embodiment of the invention, where the harmonic disturbanceis well within the PLL bandwidth, an additional integrating pole or abump (or resonant) filter is added to the loop. Assume the frequency ofthe harmonic disturbance to be at frequency w₀. The goal then isminimize this error signal within the loop, given the usual constraintsincluding loop stability. This is done by maximizing the gain of theloop at frequency w₀, without destabilizing the loop, so that the loopfollows that harmonic and minimizes the error. This is shown in FIG. 5,applied to a mixing loop.

Input signal 150 passes through bandpass filter 510. The harmonicdisturbance of interest is within the passband of bandpass filter 510.The output of bandpass filter 510 goes to mixer 520, which mixes thesignal with VCO output 570 to generate signal 525 which contains highfrequency components and baseband components (which include the phaseerror between the reference clock and the generated clock). Low passfilter 530 attenuates the high frequency components, leaving mostly thesignal with phase error 540. This signal 540 is passed to loop filter550 which provides stability of the PLL, and to harmonic corrector 560.While loop filter 550 functions as is known in the art, harmoniccorrector 560 in this embodiment passes the mixing product caused by theharmonic disturbance at w₀. The outputs of loop filter 550 and harmoniccorrector 560 control VCO 570. The output 580 of VCO 570 drive mixer520, and provide reference output 590. Harmonic corrector 560 may beswitched in and out of the loop by switching its input, output, or both.This form of harmonic correction may also be applied to harmonic lockingloops and Costas loops, mixing or phase detector loops, analog ordigital (single rate or multi rate).

In this embodiment, as in others, a multiplicity of filters is present.In FIG. 5 for example, low pass filter 530 is followed by loop filter550 and harmonic corrector 560. While FIG. 5 shows these three elementsseparately, other topologies are possible. Rather than implementingthese in parallel with their outputs summed as shown, it may beadvantageous to place them in series. This is shown in FIG. 6, showing aharmonic locking phase-locked loop with harmonic correction. Thisimplementation shows harmonic corrector 600 placed in series with loopfilter 350, and feeding VCO 360.

FIG. 7. Shows a mixing Costas loop with harmonic correction. Harmoniccorrector 700 feeds summer 700, which combines with the output of mixer460 to drive loop filter 470 and VCO 480. In some implementations, itmay be desirable to combine these separate filters. This is especiallytrue for digital signal processor (DSP) implementations, where it iscommon to convolve the separate filters into one equivalent filtersection, as is known to the art.

In a second embodiment of the invention, where the harmonic disturbanceis well outside the PLL bandwidth, an additional low pass or notch(anti-resonant) filter is added either to the inside of the loop or theoutside of the loop. The former is a form of internal correction, andthe later is a form of external correction.

In a third embodiment, feedforward cancellation is performed bygenerating a sinusoid at a phase and frequency so as to cancel out theharmonic disturbance. In this digital approach, an adaptive feedforwardcanceler, a matching signal is generated, and then used to cancel theresidual harmonic error. The matching signal is generated by modelingthe residual harmonic disturbance as a Fourier series, and identifyingthe relevant Fourier coefficients. Cancellation takes these coefficientsand injects the matching signal into the loop at an appropriate point toeither cancel the signal (external correction), or follow the signal(internal correction). This approach applies to any linear combinationof harmonics deemed important. Feedforward cancellation is taught inHarmonic Generation in Adaptive Feedforward Cancellation Schemes, IEEETransactions on Automatic Control, 39(9) pp 1939-1944 by Bodson et al.,1994.

In a fourth embodiment, harmonic cancellation is performed using arepetitive control scheme by using a filtered version of the residualerrors on previous rotations of the media. Repetitive control schemesare taught by Tomizuka et al., Discrete-time domain analysis andsynthesis of repetitive controllers, Proceedings of the 1988 AmericanControl Conference, pp 860-866. In this digital approach, N samples ofthe disturbance are calculated during the rotation of the media. Theharmonic portion of the disturbance it repeats with each revolution, andtherefore, with each additional N samples. Thus, by calculating andstoring N (or a filtered multiple of N) samples of the residual errors,and filtering these samples so as to maintain loop stability, theharmonic disturbance can once again either be canceled or followed bythe PLL. This structure provides a periodic integrator with period N.The periodic integrator when coupled into the feedback loop drives outall disturbances of period N.

The foregoing detailed description of the present invention is providedfor the purpose of illustration and is not intended to be exhaustive orto limit the invention to the precise embodiments disclosed. Accordinglythe scope of the present invention is defined by the appended claims.

1. A method of reducing the effects of the harmonic disturbance on aphase-locked loop comprising: reading a signal from a rotating media;recovering a reference signal from the rotating media with thephase-locked loop; applying a harmonic correction to the phase-lockedloop, the harmonic correction being generated by notch filteringharmonic content from the reference signal, the harmonic content beinginduced by rotation of the media.
 2. The method of claim 1 where thecorrection is applied to the phase locked loop continuously.
 3. Themethod of claim 1 where harmonic correction to the phase-locked loop isswitched in and out.
 4. A method of reducing the effects of the harmonicdisturbance on a phase-locked loop comprising: reading a signal from arotating media; recovering a reference signal from the rotating mediawith the phase-locked loop; adding a resonant filter to the phase lockedloop, the resonant filter increasing the loop gain of the phase-lockedloop at a harmonic disturbance, the harmonic disturbance being inducedby rotation of the media.
 5. A method of reducing the effects of theharmonic disturbance on a phase-locked loop comprising: reading a signalfrom a rotating media; recovering a reference signal from the rotatingmedia with the phase-locked loop; generating a sinusoid at a same phaseand frequency as a harmonic disturbance, the harmonic disturbance beinginduced by rotation of the media, and feeding forward the generatedsinusoid so as to cancel the harmonic disturbance.
 6. A method ofreducing the effects of the harmonic disturbance on a phase-locked loopcomprising: reading a signal from a rotating media; recovering areference signal from the rotating media with the phase-locked loop;collecting residual errors from a harmonic disturbance over one or morerotations of the media, the harmonic disturbance being induced byrotation of the media, filtering the residual errors, and feedingforward the filtered residual errors.